Sleep need, Sleep Debt and Body Clock

The three ideas in short#

• Sleep need is a number of hours, between 7 and 9 for most adults. It starts from your age and sex and moves a little toward the nights on which your device scored your sleep well.
• Sleep Debt (Czech "Spánkový dluh") is the running balance between what you slept and what you needed. Short or broken nights add to it, long enough nights pay it down, and a long run of long nights builds a small reserve. It is shown in hours: "−1h 30m" means you owe an hour and a half, "+30m" means you hold a half hour in reserve.
• Body Clock (Czech "Vnitřní hodiny") is a 0 to 100 index of regularity. A higher number means the same length of night, at about the same clock time, day after day. It does not say whether your hours are good ones, only how steady they are.

None of this is a medical result. MyBodyAI is a wellness app, and these numbers describe sleep your device recorded. If you sleep badly for weeks or feel unwell, see a doctor.

Where you see them#

• Sleep tab, the night panel: under the length of the night it prints "need ~7h 30m" (Czech "potřeba ~7h 30m") and how far the night is from your 30-day mean length ("+8 % vs your 30-day mean").
• Sleep tab, foot of the night panel: the pill "Sleep debt" with the debt and its word ("−1h 30m · mild"). It opens the Sleep Debt card with seven bars. The pill "Tomorrow’s recommendations" shows "in bed around 22:42" or, when we cannot trust a bedtime, "aim for ~7h 30m", and opens the evening card.
• Sleep tab, cards: Body Clock ("Body Clock · 14 days"), "Week and weekend", "Naps", and at the foot "Sleep → Body Status", which shows Sleep Quality, Debt, Morning energy and Recovery as one chain into your Body Status.
• Overview: the card "Sleep debt" with the same hours, the same seven bars and "vs 7h 30m optimum: +51m" for last night.
• Body Status: Sleep Debt is one of the signals it reads, and its sentence can name your debt (Body Status).

The Sleep tab and the Overview card are Premium (Free and Premium).

Your sleep need#

How we set it#

The need is a number of hours of sleep, not of time in bed. We start from your age and, for women, a small allowance, then check it against the range that the sleep societies recommend for your age (AASM and Sleep Research Society 2015, National Sleep Foundation 2015, Hirshkowitz 2015).

• If we do not know your age (no birth date in your profile), we read you as an adult and start at 7.5 h.
• For women we add 15 minutes (0.25 h) to the starting point. Where your profile does not state a sex, nothing is added.
• The result is rounded to hundredths of an hour and always stays inside the range in the third column. Personal habit can tune the need, it cannot take it under the minimum for your age.

A 35-year-old man starts at 7.5 h and a 35-year-old woman at 7.75 h (7h 45m). A 50-year-old man starts at 7 h and a 50-year-old woman at 7.25 h (7h 15m).

The personal blend#

Once we have enough of your history we let your own nights move the need a little. The rule is deliberately small and slow:

1. We look at the last 90 days and need at least 14 days of data in them.
2. From those days we keep the nights on which your device sent a sleep score of 75 or more. A score that we modeled ourselves does not count here, because it would only echo the length of the night.
3. With at least 5 such nights, we average their length and keep that average between 5.5 h and 10 h.
4. The need becomes 60% the starting point plus 40% that average, and is then pulled back inside the recommended range from the table.

The idea is that a good night for you tells us roughly how long you sleep when you sleep well. A device score of 75 can also reward an efficient short night, which is why the blend is limited to 40% and why the range acts as a floor.

You cannot type a different need. It follows from your profile (birth date and sex, see Settings) and from your device's scores, and it can drift by a few minutes from one day to the next as the 90-day window moves.

Where the need is used#

• Sleep Quality measures the length of the night against it (Sleep Quality).
• Sleep Debt counts every night against it (below).
• Tonight's goal and the bedtime are built on it (bedtime advice).
• The short-night test of Body Status compares last night with it, and counts it as at least 6h 40m so that a night under 6 hours is always short (short night).
• Training Readiness uses the same test (readiness).

Sleep Debt#

What the number means#

Sleep Debt answers one question: after the recent nights, how many hours short of your need are you, and how many are banked? It is a running balance, not the shortfall of last night alone. Seven hours of sleep against a need of 7h 30m costs half an hour, and a few such nights in a row add up.

The balance is built from your last 30 days of nights, oldest first. A night that is not in your records is skipped, and it changes nothing. Debt older than the window is not counted. The model is based on sleep-restriction studies that found cognitive costs adding up over days (Van Dongen 2003) and recovery taking several nights, not one (Banks 2010, Belenky 2003, Kitamura 2016). The hour limits and the exact rates below are our own calibration, not published constants.

How one night changes the balance#

Each night does three things, in this order.

1. The old balance fades a little. How much depends on how much you slept, as a share of your need (naps counted, see naps):

A night under 60% of the need (about 4h 30m at a need of 7h 30m) lets your debt keep every hour. A night that is long enough lets a quarter of it go.

2. The night's shortfall or surplus is worked out. The shortfall is your need minus the hours slept (naps credited), plus the quality penalty and minus the deep-sleep allowance described below.

3. The shortfall is added, or the surplus is subtracted.

• A shortfall is added to the debt. The nearer the debt is to its ceiling of 15 hours, the less of a new shortfall is added: the share added is 1 minus the debt divided by 15. At a debt of 3 hours, 80% of a new shortfall is added.
• A surplus (a night longer than your need) pays the debt down, but at most 1.5 hours per night count, and the payment is the surplus times the larger of two shares: the debt divided by 15, or 15%. A long night pays more when the debt is large and never less than 15% of its surplus.

The balance never goes above 15 hours of debt or above 3 hours of reserve. Every night is also capped at 12 hours of sleep, because a longer record is more likely a sensor error.

Quality and deep sleep#

Not every hour counts the same, so a night's shortfall is adjusted:

• A poor sleep score adds a penalty. When the night's sleep score is under 50, the penalty is (50 minus the score) divided by 100 hours, at most 0.5 h. The score is your device's, or the estimate we make from the night when the device sends none.
• Time awake adds a penalty. When more than 15% of your time in bed was awake (an efficiency under 85%), the penalty is (the awake share minus 0.15) times 2 hours, at most 0.2 h. The efficiency is the same one shown on the Sleep tab (efficiency). When efficiency cannot be measured for the night, no penalty is added, because a missing measurement is not a bad night.
• The two penalties together are at most 0.5 h per night. They also shrink the surplus of a long night: a long night with a poor score or a lot of waking pays the debt back less.
• Strong deep sleep takes a little off a short night. When more than 20% of your measured sleep was deep sleep, the shortfall is reduced by (the deep share minus 0.20) times 3 hours, at most 0.4 h. This applies only to a night that fell short of the need, never below zero, and never to the quality penalty. The idea is that the brain makes up some lost sleep depth when it sleeps short. Nights whose stages are physically implausible (a device that over-reports deep sleep) get no allowance.

How fast extra sleep pays it back#

Debt clears slowly on purpose. The research behind the model found that one long night does not erase a week of short ones (Banks 2010: a single 10-hour night after five nights of 4 hours was not enough). In practice:

• a night at your need keeps 75% of the debt, so about a quarter fades every day you sleep enough;
• a night 1 hour over the need pays at least 0.15 h on top of that;
• a night between 60% and 100% of the need lets only 2% to 15% fade, and adds its own shortfall;
• a night under 60% of the need lets nothing fade.

Naps#

Daytime sleep that your device records as a nap counts toward the day's sleep, because the recommended hours are stated per 24 hours. The rules:

• A nap is credited for at most 2 hours a day.
• Nothing is credited on a day whose naps are longer than its night. That looks like one sleep period that the device split in two (a shift worker's day, for example), not like a nap.
• Nothing is credited on a day with no measured night.

The Naps card ("Naps" / "Šlofíky") shows the total, how many naps, their average length and a strip of the last days. It hides itself when your device reports none (naps and Sleep Quality). On the Overview, a line such as "and a 2h 25m nap, 2h 00m of which counts toward the debt" says how much of a long nap was credited.

A night that is not recorded#

• A night that has not arrived yet. Data reaches us late as a matter of routine. For the day being lived and the day before it, a night that is missing whole can be scored as your usual night, so that a delivery delay does not raise your debt. The day is then marked provisional until the real night comes (a night that has not arrived).
• A night with no record, but a worn watch. If your Garmin watch with Body Battery was worn (it has steps, stress or a resting rate) and the overnight recharge was under 40% of your own usual recharge, and the battery did not climb from the evening to the morning by that much either, we count the night as an all-nighter: one full night of need is added to the debt, with no fade. The usual recharge is the median over your nights with sleep (at least 3). Any other night with no record is skipped, because "no data" is not "no sleep".
• When a worn watch has no night, the Sleep tab says "Last night’s sleep was not recorded" and shows the last night that was recorded. If the watch also showed no overnight recovery, it adds "No overnight recovery was detected. Below is your last recorded night, not today."

A reserve#

When the debt is cleared and you keep sleeping more than you need, the surplus is banked as a reserve of up to 3 hours, with the bank filling more slowly as it nears the cap. It fades the way a debt does and cushions the next short night before any debt appears. A reserve is shown with a plus: "+1h 30m". It is a buffer, not a stronger kind of good, so on the 0 to 100 gauge a reserve counts as 100, the same as no debt.

The gauge and the words#

Besides the hours, the debt has a 0 to 100 gauge (higher is better) that drives its color and its word. Hours map to the gauge along fixed steps, and every step ends where a word changes, so a number, a color and a word always describe the same amount of sleep.

The gauge is only a rendering of the hours. Where the app needs a higher-is-better 0 to 100 figure for the debt, it is called "Sleep balance" ("Spánková bilance").

How the hours are printed: in the app's own sign, minus is debt, plus is reserve. Hours are rounded to the nearest ten minutes, and anything under 5 minutes either way prints as a bare "0". The card under the number says one of: "A run of earlier bedtimes pays it down." (over 1 hour), "Aim for longer, consistent sleep." (over 5.5 hours), "You hold a healthy sleep balance." (up to 1 hour), or "Need more nights to compute."

A worked example#

Your need is 7h 30m and the balance is 0 before night 1. You then sleep like this.

On the screen, night 3 reads "−2h 40m" and the word "Mild"; night 7 reads "−1h" and "Very low". Two nights of 8h 30m did not clear a debt of 2.7 hours. Most of the way back came from the fade, not from the surplus.

A second example shows the penalties. Your need is 7h 30m, you sleep 6h 00m with a device score of 40 and 1h 30m awake in bed. The shortfall is 1.5 h. The score of 40 adds (50 − 40) / 100 = 0.1 h. The awake share is 1.5 / 7.5 = 20%, so the awake penalty is (0.20 − 0.15) × 2 = 0.1 h. The night adds 1.7 h to the debt.

When Sleep Debt is not shown#

• It needs sleep length on at least 3 days in your records. Until then the card says "Need more nights to compute." and the pill says "not worked out yet".
• The newest night must be fresh, which means dated today or yesterday (within 36 hours). If it is older but you have at least 3 nights in the last 7 days, the debt is shown as an assessment of the week with lower confidence. With fewer, it is withheld.
• The strip of bars needs at least two scored nights in the last seven days. Without them the card shows the number alone.

What the debt changes#

• Body Status: 3 hours or more is listed as a thing to watch on a Steady day and keeps the day from Strong, under 1 hour is needed for Peak Day, and over 6 hours makes the day Recovering (Recovering, Strong, Peak Day).
• Training Readiness: debt above 2 hours takes 3 points per extra hour, at most 15 (readiness).
• Tonight's goal and bedtime: see the next section.
• Daily goals: a debt above 3 hours nudges the automatic movement goals down a little (daily coach).
• The bedtime mission: the night counts as on plan when you fall asleep within 30 minutes either side of the planned bedtime, and only a bedtime we trust (below) is a plan (Progress).

Tonight's goal and the bedtime advice#

Tonight's sleep goal#

Tonight's goal is your need plus a part of your debt:

• With a debt of 1 hour or less, the goal is your need rounded to the nearest half hour.
• Above 1 hour, the goal is your need plus 35% of the debt, at most 1.5 hours more than the need, and the result is rounded to the nearest half hour.

You see this goal as "Sleep goal" ("Cíl spánku") among your daily goals, in the sleep tips, and in the advice of Body Status on an Alert or Recovering day ("... and 7h 30m of sleep tonight"). The goal is a number of hours of sleep. It is not an instruction to lie in bed for that long.

The advised bedtime#

The bedtime is a clock time worked out backward from your wake time:

bedtime = wake time − need ÷ efficiency − debt shift

• Wake time: see the next section.
• Need ÷ efficiency: your need is time asleep, but a bed holds some time awake. We take your own median efficiency over the last 30 days (sleep length divided by the time from bedtime to wake time), from nights that have both clock times, a window of 3 to 16 hours and no more sleep than window. It needs at least 7 such nights, otherwise we assume 100%. It is kept between 85% and 100%, so a poor sleeper is never told to lie in bed longer than a good sleeper would need.
• Debt shift: 0.3 hours (18 minutes) for each hour of debt, at most 1 hour. A reserve or no debt shifts nothing. We stop at an hour because the evening hours just before your usual bedtime are the hardest to fall asleep in (Lavie 1986), so a much earlier bedtime mostly buys time awake in bed.

When the shift is not zero, the evening card says so: "Aim to be in bed around 22:42", "From your data (wake ~06:30). 18 min earlier to pay back your sleep debt." When the bedtime includes time you lie awake (15 minutes or more), it adds "Including the time you lie awake."

The Body Clock card draws your rhythm's window from the same wake time and need but without the debt shift, because that window describes your rhythm, not tonight's repayment (see Body Clock).

Which wake time we use#

1. A fixed wake time you set always wins (setting your own schedule).
2. Otherwise we read your measured wake times from the last 60 days. We take the wake times of the same weekday as the morning you are heading for when we have at least 3 of them, otherwise all days, and average them round the clock, so a wake around midnight does not give nonsense.
3. With neither, we use 06:00, and then we do not print a bedtime.

How sure we are depends on the nights we averaged. Seven or more nights that agree to within about an hour are high confidence. Four or more that agree to within about two hours are medium. Anything looser is low.

When we show a bedtime#

We print a clock time only when we can stand behind its wake time: you set it yourself, or it comes from your own nights with high or medium confidence. Otherwise the card says "Aim for ~7h 30m of sleep" with "Set your wake time for an exact bedtime." The same rule decides what the Progress tab, the energy map and the bedtime mission print, so the app never states one time on one tab and none on another. An account that has never recorded a night gets no bedtime at all, instead of a made-up 22:30.

Shift work and changing clocks#

An average of wake times breaks for people whose weeks are not weeks. When your wake times fall into two or three habits (an early shift, a free-day wake, a wake after a night shift), we look for a cycle of 1 to 28 days that repeats them:

• It needs at least 8 nights, and each habit must have at least 3 nights behind it.
• The cycle must match your nights at least 75% of the time and beat "always the commonest habit" by at least 15 points, and it needs two full cycles of evidence. Otherwise we keep the plain average rather than a confident wrong guess.
• A date's habit is the vote of the same place in the previous one to four cycles (at least 60% of the vote). Its wake and bedtime are the recent averages of that habit's own nights, with newer nights weighing more (the weight halves every 21 days).
• A bedtime between 05:00 and 15:00, the sleep after a night shift, is kept as it is instead of being set to wake minus the need.

When your newest nights agree with each other and sit three hours or more away from the older ones (a new job, a new time zone), the newer clock replaces the older history, so the advice follows your new life within days. A smaller change needs about a week. A bedtime you plan for the night on the energy map (the day plan) replaces the advised one.

Setting your own schedule#

On the Sleep tab, the evening card has a button "Schedule" ("Čas spánku"). It opens "Sleep schedule" ("Spánkový režim") with the question "Which wake time anchors your bedtime":

• "Automatic" ("Automaticky"): "We estimate your wake time from your measured nights."
• "Fixed time" ("Pevný čas"): you enter "Wake" ("Vstávám") and, optionally, "Bed (optional)" ("Uléhám (nepovinné)") as HH:MM. The wake time then anchors the advised bedtime. The optional bed time is used for the sleep window drawn on the energy map.

After you save, the Sleep tab loads again with the new anchor.

Body Clock#

What it measures#

Body Clock is a 0 to 100 index of how regular your sleep is, over 14 calendar days. It is the same idea as the regularity indexes of the sleep research: irregular sleep timing is associated with a delayed body clock and with metabolic risk (Lunsford-Avery 2018, Phillips 2017). It scores steadiness. It does not score whether your hours are the best ones for your body.

The card reads "Aligned" ("V souladu") from 80, "Slightly off" ("Mírný posun") from 60 to 79, and "Irregular" ("Nepravidelný") below 60. The number in the ring is the same whole number the words are judged on.

The parts#

The weights are relative: the score is the weighted average of the parts that exist for you. With the Polar gate present it counts 20 of 120, about a sixth, and the other parts shrink to make room.

• Length: 100 − (the standard deviation of your nightly sleep seconds ÷ 120). A spread of 30 minutes scores 85, 1 hour scores 70.
• Deep sleep: 100 − (the standard deviation of deep-sleep seconds ÷ 54). Deep sleep swings more from night to night than total sleep, so its divisor is smaller. A spread of 20 minutes scores 78. Only nights with at least 5 minutes of deep sleep count, because a shorter episode is most likely a sensor error. This part needs 5 nights.
• Bedtime and wake time: 100 − (the spread in minutes ÷ 1.2). A spread of 30 minutes scores 75, 60 minutes scores 50, 120 minutes scores 0.

Each part needs 5 nights with the value in the 14 days. Sleep length needs 5 nights or the whole index is withheld. Nights longer than 12 hours count as 12.

How the clock spread is measured#

Bedtime and wake time are times on a 24-hour circle, so 23:50 and 00:10 are 20 minutes apart. The spread is worked out like this:

1. We find your usual clock time: the middle of your nights, nudged to the average of the nights within three hours of it.
2. Each night's distance from the usual is capped at 3 hours.
3. The spread is the root of the average of the squared distances.

The cap means that one odd night, for example one daytime sleep after a night shift among thirteen steady nights, costs part of the score and not all of it. A steady rhythm with no odd night is scored as if there were no cap.

Polar's sleep gate#

Polar watches model a window in which your body falls asleep most easily. When your Polar data carry it, we compare your measured onset with that window on each night:

The gate part is the average of those nights and needs 5 nights with both an onset and a window. The window itself is Polar's model, not a measurement and not our arithmetic. It is shown on the Body Clock card as "Sleep gate (Polar)" with the note "model", and it is never scored alone. Oura publishes an "ideal bedtime" window, which we show as "Ideal bedtime (Oura)" with the same note, but it is not scored.

Minimum data and damping#

Body Clock needs 7 days of history (and at least one of sleep timing, sleep length or sleep stages), plus the 5 nights of sleep length above. With few parts the index could swing on very little, so it is pulled toward 50:

• 4 or more parts: the score as computed;
• 3 parts: it keeps 80% of its distance from 50;
• 2 parts: it keeps 60%;
• 1 part: nothing is damped, since there is nothing to average.

Below 60, the daily tips can say "Irregular sleep timing, try going to bed and getting up at the same times." and suggest a steadier bedtime.

The Body Clock card#

Title "Body Clock · 14 days" ("Vnitřní hodiny · 14 dní"). It hides itself when there is no score, no rhythm window and no device rows. Its rows:

• Your rhythm’s window ("Okno Vašeho rytmu"): your wake time minus one normal night (need ÷ efficiency), to your wake time. It is drawn only when the wake time passes the rule for showing a bedtime.
• Actual sleep ("Skutečný spánek"): when the shown night started and ended.
• Bedtime deviation ("Odchylka ulehnutí"): the night's start against the rhythm's bedtime, in minutes, plus later and minus earlier. Up to 30 minutes reads as good, up to 60 as fair, above that as low. It is measured against the rhythm bedtime and not the advised one, so you are not marked off-rhythm for keeping your usual hour while you owe sleep.
• Sleep gate (Polar), Hit the gate ("Trefa do okna"), Ideal bedtime (Oura), In bed in the window ("Ulehnutí v okně"), Day's alertness (Polar) ("Bdělost dne (Polar)", 0 to 10): device rows that appear only if your device sends them.

When the shown day has no bedtime or wake time, the card says "No bedtime or wake time recorded for this day. The score comes from 14-day regularity."

The typical week#

At the foot of the card is "Typical week" ("Typický týden"): one bar for each evening from Monday to Sunday. A night is filed under the evening it began, so the night from Friday to Saturday is the Friday bar. Each bar is the median of up to a year of your nights for that evening: drawn around the median middle of the night and as long as the median time in bed, with the median hours asleep at the right. Bars stacked under each other mean a steady week. A Friday and a Saturday shifted to the right is a typical weekend.

• An evening needs at least 4 nights in the year to be drawn, and at least 4 evenings must be drawn for the picture to show.
• The dashed band is "rhythm window", the window from the card above.
• The footer says "Median of 62 nights (…), hours asleep on the right."
• Hover (mouse) or tap a bar for "Friday night · 23:10 to 06:40 · 7 h 20 min asleep · 9 nights".
• The score above still uses only 14 days. A long window would mix last spring's rhythm into today's.

Week and weekend#

The card "Week and weekend" ("Týden a víkend") puts your typical weekday night beside your typical weekend night. Weekend nights here are the nights that end on a Saturday or Sunday morning. It reads up to a year of nights.

• It needs at least 8 weekday nights and 4 weekend nights with both a start and an end, and says "Weekday nights 5 of 8 · weekend 2 of 4" while collecting.
• Social jetlag is how far the middle of your night moves at the weekend (Wittmann 2006). Under 5 minutes the card says it falls at the same time as on weekdays and "your rhythm holds all week". Under 30 minutes it says it moves only a few minutes and that "your rhythm holds all week". From 30 minutes it says "The middle of your night falls 50 min later at the weekend than on weekdays. This is called social jetlag." If you also sleep at least 30 minutes longer at the weekend, it adds that you sleep less during the week and make up for it.
• Length: within 10 minutes the card says you sleep the same on both kinds of day.
• Late evenings: when your own weekday nights show it, a second sentence says what a later evening costs, for example "When you go to bed later than usual on an evening before a weekday (typically 1 h), the morning does not make it up: you sleep 45 min less." It is printed only for a clear result: at least 14 usable nights, at least 5 nights of the later kind, bedtimes 30 minutes or more later than your usual, a permutation test below 0.025 and a difference of at least 15 minutes, with the wake time following by less than half of the bedtime shift. A night more than 3 hours from the usual is a different schedule and is left out. Otherwise the card says nothing about why rather than guessing.

When these update#

• Sleep need is recomputed with the rest of your day, from your profile and the 90 days behind the day.
• Sleep Debt changes when a night arrives. It does not move through the day. Results are reused for up to 5 minutes, and a sync or a profile change refreshes them sooner (sync).
• Body Clock changes when a night arrives, because the 14-day window moves.
• The bedtime advice is for tonight and changes with the debt and with the wake time of tomorrow's weekday.
• Past days are recalculated from the data we hold for that day when you open them, so late data or a changed calculation can move them a little (how MyBodyAI works).

Free and Premium#

Since 1 November 2026 the free plan shows today's Body Status with its sentence and advice, and the safety alerts. The Sleep tab and the Overview's "Sleep debt" card are Premium. They hold the Sleep Debt card and bars, Body Clock, the bedtime advice, the Naps card and the week cards. On the free plan Body Status can still mention your debt in its sentence ("a sleep debt of about 3h 30m") and name tonight's goal in its advice. Premium includes the 7-day trial, and your data keeps arriving whatever the plan (plans).

Energy pacing mode#

The calculation of need, debt and Body Clock does not change in the energy pacing mode, and the sleep goal stays when the movement goals go (energy pacing).

What you can do#

• Keep your wake time steady first. It anchors the bedtime advice and the Body Clock, and the bedtime follows.
• Shift your bedtime in small steps, not by an hour at once. The advised shift is capped at an hour for that reason.
• Pay a debt back over days. A hard debt does not clear in one long night.
• Wear the device to bed. A night with no record is skipped, not forgiven, and a worn watch with no recharge counts as a night without sleep.
• Read the week. One late night is one point on the strip, and "Week and weekend" tells you whether it is a pattern.
• If you sleep badly most nights, or feel exhausted despite long nights, see a doctor. These numbers are an overview of your habits and not a medical assessment.

MyBodyAI is a wellness app, not a medical device. Sleep need, Sleep Debt and Body Clock describe signals from your device. They do not diagnose.

Frequently asked questions#

Why does my sleep need differ from what I thought, or from another app?#

Because ours starts from your age and sex, stays inside the recommended range for your age, and moves up to 40% of the way toward the length of your own good nights (device score 75 or more). Another app may use a flat 8 hours. You cannot type a need in, but you can change your birth date and sex in your profile if they are wrong (Settings).

Why does my Sleep Debt not clear after two long nights?#

It is meant to be slow. A night at your need lets about a quarter of the old debt fade, and a surplus pays at least 15% of its hours, at most 1.5 hours count per night. Studies of sleep restriction found that recovery takes several nights (Banks 2010, Kitamura 2016). The worked example above goes from 2.7 hours to 1.0 hour in three nights, two of them long.

Why is my debt larger than the shortfall of last night?#

Because it is a running balance over 30 days, and because a poor sleep score or a lot of time awake adds up to half an hour to a night's shortfall. Open the Sleep Debt card and read the seven bars: the last bar is the number above it.

Why did my debt go up on a night I slept enough?#

Usually one of three reasons. The sleep score was under 50, or you were awake more than 15% of your time in bed, and the penalty outweighed a small surplus. Or the night has not arrived, and we scored the day with your usual night (the day is marked provisional). Or your watch had no overnight recharge and no sleep was recorded, so we counted it as a night without sleep.

Does a nap count toward my debt?#

Yes, up to 2 hours a day, and not on a day when your naps are longer than your night. A nap is not part of the night's length on the Sleep tab. It is a separate record on the Naps card (naps and Sleep Quality).

Why is my Body Clock low when I sleep enough?#

Body Clock measures how steady your sleep is, not how much you sleep. A 14-day spread of 1 hour in bedtime and wake time scores 50 on those parts even if every night is 8 hours. Weekends, shifts and travel move the clock. A single odd night is limited to 3 hours of distance, so one night costs part of the score, but a pattern costs more.

Why does the app ask for a different bedtime than I keep?#

The advised bedtime is your wake time minus your need, divided by your own efficiency, minus up to one hour for a debt. If you keep your usual bedtime and owe sleep, the advice is earlier than your habit by design, and the card says how many minutes are for the debt. The Body Clock card compares you with your rhythm bedtime, which has no debt shift in it.

Why is there no bedtime on my card?#

We print a bedtime only when we trust its wake time: you fixed one, or we have at least 4 recorded nights (of the same weekday, or of all days) that agree to within about 2 hours. Until then you see a sleep goal in hours instead. Recording a few nights, or setting a "Fixed time" in the sleep schedule, brings the bedtime back.

Does the app diagnose a sleep disorder?#

No. It counts your hours against a need, compares clock times from night to night and suggests a bedtime. It cannot tell insomnia or sleep apnea from a bad week. If your sleep worries you, see a doctor.